Chemically stable solid lithium ion conductor

ABSTRACT

The present invention concerns chemically stable solid lithium ion conductors, processes for their production and their use in batteries, accumulators, supercaps and electrochromic devices.

This application is divisional of 10/591,714 filed Sep. 6, 2006, whichis a 35 U.S.C. 371 National Phase Entry Application fromPCT/EP2005/002255, filed Mar. 3, 2005, which claims the benefit ofGerman Patent Application No. 10 2004 010 892.7 filed on Mar. 6, 2004,the disclosure of which is incorporated herein in its entirety byreference.

The present invention concerns chemically stable solid ion conductors inparticular lithium ion conductors, processes for their production andtheir use in batteries, accumulators and electrochromic devices.

Mobile energy stores with high energy densities (and high powerdensities) are required for numerous technical devices, above all formobile telephones and portable computers (e.g. notebooks). In thisconnection rechargeable chemical energy stores, especially secondarybatteries and super-capacitors are of supreme importance.

The previous highest energy densities in the range of 0.2 to 0.4 Wh/cm³are nowadays commercially achieved with so-called lithium ion batteries.These usually consist of a liquid organic solvent (e.g. EC/DEC)containing a lithium salt (e.g. LiPF₆), an anode made of graphite withintercalated lithium and a cathode made of lithium cobalt oxide wherethe cobalt may be partially or completely replaced by nickel ormanganese.

It is generally known that the service life of such lithium ionbatteries is quite limited and hence they often have to be replaced evenduring the lifetime of the device to be supplied. Moreover, it isgenerally expensive to get replacements and disposal of the oldbatteries is problematic since some of the ingredients are notenvironmentally friendly.

In operation the batteries of the prior art prove to be not sufficientlypowerful for many applications (e.g. offline operation of a notebook fora maximum of a few hours). The batteries are chemically unstable whenelectrodes are used that enable higher voltages of for example 5 V ormore; the organic electrolyte components start to decompose at voltagesabove 2.5 V. The liquid electrolyte is in any case a safety hazard: inaddition to the risk of leakage, fire and explosion, the growth ofdendrites is also possible which can result in a high self-discharge andheating.

Liquid electrolyte batteries are basically unsuitable for some technicalobjectives because they must always have a minimum thickness and thuscan only be used to a limited extent as thin energy stores e.g. on chipcards.

Solid lithium ion conductors such as Li_(2,9)PO_(3,3)N_(0,46)(Li_(3−x)PO_(4−y)N_(y), LIPON) are also known and have been used on alaboratory scale in thin layer batteries. However, these materialsgenerally have a considerably lower lithium conductivity than liquidelectrolytes. Solid lithium ion conductors having the best ionconductivities are Li₃N and Li-β-alumina. Both compounds are verysensitive towards water (moisture). Li₃N already decomposes at a voltageof 0.445 V at room temperature; Li-β-alumina is chemically unstable.

Lithium ion conductors having a garnet-like structure were presented inthe paper “Novel fast lithium ion conduction in garnet-type Li₅La₃M₂O₁₂(M=Nb, Ta)” by Thangadurai et al. (J. Am. Ceram. Soc. 86, 437-440,2003).

Garnets are orthosilicates of the general composition A₃B₂(SiO₄)₃ inwhich A and B represent eight-coordinate or six-coordinate cationpositions. The individual SiO₄ tetrahedrons are connected together byionic bonds with the interstitial B cations. The compounds of theformula Li₅La₃M₂O₁₂ (M=Nb, Ta) have a garnet-like structure. Theycrystallize in a cubic symmetry with the lattice constant a=12.797 Å or12.804 Å respectively for the corresponding compound in which M=Nb orTa. Compared with the ideal garnet structure there is an excess of 16lithium ions per formula unit. The La³⁺ and M⁵⁺ ions occupy theeight-coordinate or six-coordinate positions whereas lithium ions occupypositions having a six-fold coordination. The similarity between theideal garnet structure and Li₅La₃M₂O₁₂ is due to the fact thatalkaline/rare earth metal ions occupy the dodecahedral (eight-)coordinate positions and M atoms occupy the six-coordinate positions.The main difference in the structures is due to the fact that Sioccupies the position with the four-fold oxygen coordination in theideal garnet structure whereas in the garnet-like Li₅La₃M₂O₁₂ Lioccupies the highly distorted octahedrical positions. The garnet-likestructure has two types of LiO₆ octahedra; of these Li(I)O₆ is moredistorted than Li(II)O₆. MO₆ octahedra are surrounded in a cubicalmanner by six LiO₆ octahedra and two vacant lithium positions. Thevacant positions are arranged along the axes between the neighbouringMO₆ octahedra.

The garnet-like Li₅La₃M₂O₁₂ compounds have a significant lithium ionconductivity. In particular it was demonstrated on thetantalum-containing compound Li₅La₃Ta₂O₁₂ that volume conductivity andgrain-boundary conductivity in the garnet-like structure tend to be of acomparable order of magnitude. Hence the total conductivity is extremelyhigh and even above that of Li-β-alumina or of Li₉AlSiO₈ but stillconsiderably below the conductivities of LISICON or Li₃N.

The object of the present invention was to provide improved solid ionconductors having a high ion conductivity, a low electronic conductivityand a high chemical stability. In particular the object of the inventionwas to provide improved lithium ion conductors.

It was found that materials having a garnet-like structure have anextremely high ionic conductivity. The novel solid ion conductors areformally derived from the already known garnet-like structures of thecomposition Li₅La₃M₂O₁₂. Surprisingly garnet-like structures having aconsiderably improved ion conductivity are produced from this compoundby aliovalent substitution.

Aliovalent substitution is understood as the substitution of an ion byan ion of another oxidation state and the resulting charge compensationthat is required can be achieved by cation vacancies, anion vacancies,interstitial cations and/or interstitial anions.

Starting with the known garnet-like structures Li₅La₃M₂O₁₂ theconnectivity of the network can be increased and the number of availablevacant positions can be varied according to the invention by aliovalentsubstitutions. In this connection the La³⁺ positions are preferablyaliovalently substituted for example by divalent cations. The chargecompensation can preferably be by means of Li⁺ cations. The conductivityof the structure can be made-to-measure by suitable doping.

Furthermore any other elements or combinations of elements can be usedaccording to the invention instead of Li, La, M and O. It is possible toobtain any ion conductors by partial or complete formal substitution ofthe Li cations by other metal cations and in particular by alkali ions.The solid ion conductors according to the invention are characterized bythe garnet-like structure that is described in detail above.

Hence the present invention provides a solid ion conductor having agarnet-like crystal structure which has the stoichiometric composition

L_(5+x)A_(y)G_(z)M₂O₁₂

wherein

L is in each case independently an arbitrary preferably monovalentcation,

A is in each case independently a monovalent, divalent, trivalent ortetravalent cation,

G is in each case independently a monovalent, divalent, trivalent ortetravalent cation

M is in each case independently a trivalent, tetravalent or pentavalentcation, 0<x≦3, 0≦y≦3, 0≦z≦3 and

wherein O can be partially or completely replaced by divalent and/ortrivalent anions such as e.g. N³⁻.

Within a structure of this formal composition L, A, G and M can each bethe same or different.

L is particularly preferably an alkali metal ion for example Li⁺, Na⁺ orK⁺. In this connection combinations of different alkali metal ions for Lare also especially possible.

A represents an arbitrary monovalent, divalent, trivalent or tetravalentcation or any combinations thereof. Divalent metal cations can bepreferably used for A. Alkaline earth metal cations such as Ca, Sr, Baand/or Mg as well as divalent transition metal cations such as e.g. Znare particularly preferred.

G represents an arbitrary divalent, trivalent, tetravalent orpentavalent cation or any combinations thereof. Trivalent metal cationscan be preferably used for G. G is particularly preferably La.

M represents an arbitrary divalent, trivalent, tetravalent orpentavalent cation or any combinations thereof. Pentavalent cations canbe preferably used for M. M is also preferably a transition metal, whichis preferably selected from Nb and Ta. Other examples of suitablepentavalent cations are Sb and V. When selecting M it is advantageous toselect transition metal ions which have a high stability towardsreduction. M is most preferably Ta.

In a structure of the above composition O²⁺ can be completely orpartially replaced by other anions. For example it is advantageous tocompletely or partially replace O²⁻ by other divalent anions. Inaddition O²⁻ can also be aliovalently substituted by trivalent anionswith a corresponding charge compensation.

Furthermore in the above composition 0<x≦3, preferably 0<x≦2 andparticularly preferably 0<x≦1; 0≦y≦3, and 0≦z≦3. The stoichiometricratio of the components is selected in such a manner that an overalluncharged garnet-like structure is present.

In a preferred embodiment of the present invention L is a monovalentcation, A is a divalent cation, G is a trivalent cation and M is apentavalent cation. Furthermore in this preferred embodiment thestoichiometry of the compound is preferably:

L_(5+x)A_(x)G_(3−x)M₂O₁₂

wherein x is defined as above and preferably 0<x≦1.

A specialized aspect of the present invention provides a solid lithiumion conductor of the stoichiometric composition Li₆ALa₂M₂O₁₂ in which Adenotes a divalent metal and M denotes a pentavalent metal. Within astructure of this formal composition A and M can in each case be thesame or different.

A is preferably selected from alkaline earth metals, preferably from Ca,Sr, Ba and/or Mg. A can also be preferably selected from divalenttransition metals such as for example A=Zn. A is most preferably Sr orBa.

M can be any pentavalent cation for example a metal in the oxidationstate +V, M is preferably a transition metal that is preferably selectedfrom Nb and Ta. Other examples of suitable pentavalent cations are Sband V. When selecting M it is advantageous to select transition metalions which have a high stability towards a reduction by elementallithium. M is most preferably Ta.

Lithium ion conductors of the composition Li₆ALa₂M₂O₁₂ have agarnet-like crystal structure. Compared to the known compounds of thecomposition Li₅La₃M₂O₁₂, La was formally replaced by a divalent ion Aand a lithium cation and thus the proportion of lithium in the structurewas increased. As a result it is possible to use the compounds of thepresent invention to provide considerably improved lithium ionconductors.

Compared to the compounds of the prior art, the materials of thecomposition Li₆ALa₂M₂O₁₂ have an increased lithium conductivity. Forexample the lithium conductivity of Li₆ALa₂Ta₂O₁₂ (A=Sr, Ba) of 10⁻⁵S/cm at 20° C. is an order of magnitude higher than that of LIPON. Dueto the garnet structure of the compounds of the present invention whichis a 3D-isotropic structure, the lithium ion conduction is possible in 3dimensions without a preferred direction.

In contrast the electronic conductivity of the compounds of the presentinvention is negligibly small. Polycrystalline samples of the compoundsof the present invention exhibit a low grain boundary resistance suchthat the total conductivity is due almost exclusively to the volumeconductivity.

Another advantage of the materials is their high chemical stability. Thematerials exhibit in particular no detectable changes when heated incontact with melted lithium. At temperatures of up to 350° C. and directvoltages of up to 6 V there is no chemical decomposition.

According to another aspect the present invention concerns processes forproducing the solid ion conductors having a garnet-like structure. Thecompounds can be formed by reacting appropriate salts and/or oxides ofthe elements that are contained therein for example by means of a solidphase reaction. Particularly suitable starting materials are nitrates,carbonates and hydroxides which during the course of the conversion areconverted into the corresponding oxides.

In particular the present invention concerns processes for producingsolid ion conductors of the composition L_(5+x)A_(x)G_(3−x)M₂O₁₂ (e.g.Li₆ALa₂M₂O₁₂). The materials can be obtained by reacting appropriatesalts and/or oxides of A, G and M with a hydroxide, nitrate or carbonateof L in a solid phase reaction. In this case A and M are defined asabove. The divalent metal A is preferably used in the form of nitrates.In this connection Ca(NO₃)₂, Sr(NO₃)₂ and Ba(NO₃)₂ are preferred. La ispreferably used for G, which is preferably used in the form of La₂O₃. Mis advantageously used as an oxide and Nb₂O₅ and Ta₂O₅ are preferred. Lis preferably used in the faun of LOH, LNO₃ or L₂CO₃. For exampleLiOH.H₂O can be preferably used. In order to compensate a weight loss ofL (e.g. L=Li) during the heat treatment of the samples, thecorresponding salt is preferably used in an excess; an excess of 10% isfor example suitable.

The starting materials are mixed in a first step and can for example beground by zirconium oxide ball-milling in 2-propanol. The mixtureobtained in this manner is subsequently heated for several hours,preferably for 2-10 h in air at temperatures in the range of preferably400-1000° C. Temperatures of ca. 700° C. and a heat treatment period ofabout 6 hours are particularly suitable for this. A grinding process issubsequently again carried out, preferably also by zirconium oxideball-milling in 2-propanol. The reaction product is subsequently pressedat isostatic pressure into moulded pieces, for example into pellets.These are then preferably sintered for several hours, preferably for10-50 h, more preferably for 20-30 h at temperatures in a range ofpreferably 700-1200° C., more preferably 800-1000° C. Temperatures ofabout 900° C. and a heat treatment period of about 24 hours areparticularly suitable for this. In this sintering process it isadvantageous to cover the samples with a powder of the same compositionin order to avoid excessive losses of the L-hydroxide.

The solid ion conductors (e.g. lithium conductors) obtained by theproduction process of the present invention are a valuable startingmaterial as solid electrolytes.

Since the materials have an unusually high ion conductivity while havinga negligible electron conduction, they can be used as a solidelectrolyte for batteries (e.g. lithium batteries) with a very highenergy density. As a result of the high resistance of the materialstowards chemical reactions e.g. with elemental lithium and towardsconventional electrode materials, the solid lithium ion conductors ofthe present invention can for example be used practically in lithium ionbatteries.

The resistance of the phase boundary between the solid electrolyte ofthe present invention and the electrodes is also very small compared tocommon electrolyte materials. As a result batteries can be producedusing the materials according to the invention which have a relativelyhigh power (high currents). The use of the solid-state electrolytes ofthe present invention improves safety compared to the use of liquidelectrolytes. This is particularly advantageous for an application inmotor vehicles.

Another aspect of the present invention concerns the use of the solidion conductors (e.g. lithium ion conductors) in electrochromic systems(windows, screens, facades etc.) as well as for instantaneous energystorage or release in super-capacitors (supercaps). In this connectionenergy densities of capacitors of 100 F/cm³ can be achieved by using theion conductors according to the invention. Another aspect of theinvention is the use of the garnet-like solid ion conductors as sensorsfor example for numerous gases.

The solid ion conductors of the present invention can be used in theform of pellets, or as thin layers in a crystalline or amorphous form.

FIGURES:

FIG. 1 shows a unit cell of the crystal structure of Li₅La₃M₂O₁₂ (M=Nb,Ta);

FIG. 2 shows the measured conductivity of Li₆BaLa₂Ta₂O₁₂ in comparisonwith other solid lithium ion conductors. The materials according to theinvention have very high ionic conductivities that are comparable withthose of Li_(3,5)P_(0,5)Si_(0,5)O₄ or even Li₃N.

FIG. 3 shows the equilibrium electron current as a function of theapplied voltage for Li₆BaLa₂Ta₂O₁₂ obtained at 22° C. and at 44° C. byHebb-Wagner (HW) measurements with a lithium ion blocking electrodeusing lithium as a reference electrode. The measurements were carriedout in a glovebox filled with argon at an oxygen partial pressure of <1ppm.

The present invention is further illustrated by the following example.

EXAMPLE Production of Pellets of Li₆ALa₂Ta₂O₁₂ (A=Ca, Sr, Ba)

La₂O₃ (predried at 900° C. for 24 h), Nb₂O₅ and A(NO₃)₂ were mixed in astoichiometric ratio with a 10% excess of LiOH.H₂O and ground for 12 hin 2-propanol using zirconium balls. The mixture obtained was heated for12 h in air to 700° C. and subsequently again ground by balls.Subsequently the mixture was pressed into pellets at isostatic pressureand covered with a powder of the same composition to avoid excessivelosses of the lithium oxide. The pellets were sintered for 24 h at 900°C. Subsequently the conductivity and the chemical stability of theresulting solid lithium ion conductors was examined. The results areshown in table 1 and in FIGS. 2 and 3.

TABLE 1 Resistance ofLi₆ALa₂Ta₂O₁₂ (A = Sr, Ba) at 22° C. in air R_(vol)C_(vol) R_(gb) C_(gb) C_(el) σtotal E_(a) Compound [kΩ] [F] [kΩ] [F] [F][Scm⁻¹] [eV] Li₆SrLa₂Ta₂O₁₂ 18.83 3.0 × 10⁻¹¹ 3.68 8.5 × 10⁻⁹ 5.7 × 10⁻⁶7.0 × 10⁻⁶ 0.50 Li₆BaLa₂Ta₂O₁₂ 3.45 1.2 × 10⁻¹¹ 1.34 1.3 × 10⁻⁷ 1.2 ×10⁻⁶ 4.0 × 10⁻⁵ 0.40 vol: volume gb: grain boundaries

1. Use of a solid ion conductor having a garnet-like crystal structureand a higher ion conductivity than 3.4×10⁻⁶ S/cm in batteries,accumulators, supercaps, fuel cells, sensors and/or electrochromicdevices such as windows, screens and facades.
 2. The use as claimed inclaim 1, wherein said solid ion conductor is used in the form ofpellets, as a thin layer, or in a crystalline or amorphous foil.
 3. Theuse of claim 1, wherein said solid ion conductor has a stoichiometriccomposition ofL_(5+x)A_(x)G_(3−x)M₂O₁₂ and wherein 0<x≦1, L is a monovalent alkalimetal cation, A is a divalent metal cation, G is a trivalent cation, andM is a pentavalent cation.
 4. The use of claim 1, wherein the solid ionconductor has a stoichiometric composition ofL_(5+x)A_(y)G_(z)M₂O₁₂, and wherein L is in each case independently anarbitrary preferably monovalent cation, A is in each case independentlya monovalent, divalent, trivalent or tetravalent cation, G is in eachcase independently a monovalent, divalent, trivalent or tetravalentcation M is in each case independently a trivalent, tetravalent orpentavalent cation, 0<x≦2, 0≦y≦3, 0≦z≦3 and wherein O can be partiallyor completely replaced by divalent and/or trivalent anions such as e.g.N³⁻, and wherein at least one of A and G is a divalent cation.
 5. Theuse of claim 4, wherein L is selected from Li⁺, Na⁺, or K⁺, which can ineach case be the same or different.
 6. The use of claim 5, wherein L isLi⁺.
 7. The use of claim 4, wherein M is selected from transition metalions.
 8. The use of claim 3, wherein A is a divalent cation.
 9. The useof claim 8, wherein A is selected from Ca, Sr and/or Ba and wherein M isselected from Nb and Ta.
 10. The use of claim 8, wherein A is selectedfrom Sr and Ba and wherein M is Ta.
 11. The use of claim 1, wherein saidsolid ion conductor is stable towards elemental lithium at lithiumactivities corresponding to a voltage of 5V.
 12. The use of claim 1,wherein said solid ion conductor has a stoichiometric composition ofL_(5+x)AyG_(z)M₂O₁₂, wherein L is in each case independently anarbitrary preferably monovalent cation, A is in each case independentlya monovalent, divalent, trivalent or tetravalent cation, G is in eachcase independently a monovalent, divalent, trivalent or tetravalentcation M is in each case independently a trivalent, tetravalent orpentavalent cation, 1≦x≦2, 0≦y≦3, 0≦z≦3 and wherein O can be partiallyor completely replaced by divalent and/or trivalent anions such as e.g.N³⁻.
 13. A method of producing a battery, accumulator, supercap, fuelcell, sensor and/or electrochromic device having high energy density andchemical stability comprising incorporating a solid ion conductor havinga garnet-like crystal structure and a higher ion conductivity than3.4×10⁻⁶ S/cm into said battery, accumulator, supercap, fuel cell,sensor and/or electrochromic device.
 14. The method of claim 13, whereinsaid solid ion conductor is used in the form of pellets, as a thinlayer, or in a crystalline or amorphous foil.
 15. The method of claim13, wherein said solid ion conductor has a stoichiometric composition ofL_(5+x)A_(x)G_(3−x)M₂O₁₂ and wherein 0<x≦1, L is a monovalent alkalimetal cation, A is a divalent metal cation, G is a trivalent cation, andM is a pentavalent cation.
 16. The method of claim 15, wherein A is adivalent cation.
 17. The method of claim 16, wherein A is selected fromSr and Ba and wherein M is Ta.
 18. The method of claim 13, wherein thesolid ion conductor has a stoichiometric composition ofL_(5+x)A_(y)G_(z)M₂O₁₂, and wherein L is in each case independently anarbitrary preferably monovalent cation, A is in each case independentlya monovalent, divalent, trivalent or tetravalent cation, G is in eachcase independently a monovalent, divalent, trivalent or tetravalentcation M is in each case independently a trivalent, tetravalent orpentavalent cation, 0<x≦2, 0≦y≦3, 0≦z≦3 and wherein O can be partiallyor completely replaced by divalent and/or trivalent anions such as e.g.N³⁻, and wherein at least one of A and G is a divalent cation.
 19. Themethod of claim 18, wherein M is selected from transition metal ions.20. The method of claim 18, wherein A is selected from Ca, Sr and/or Baand wherein M is selected from Nb and Ta.
 21. The method of claim 18,wherein L is selected from Li⁺, Na⁺, or K⁺, which can in each case bethe same or different.
 22. The method of claim 21, wherein L is Li⁺. 23.The method of claim 13, wherein said solid ion conductor is stabletowards elemental lithium at lithium activities corresponding to avoltage of 5V.
 24. The method of claim 13, wherein said solid ionconductor has a stoichiometric composition ofL_(5+x)AyG_(z)M₂O₁₂. wherein L is in each case independently anarbitrary preferably monovalent cation, A is in each case independentlya monovalent, divalent, trivalent or tetravalent cation, G is in eachcase independently a monovalent, divalent, trivalent or tetravalentcation M is in each case independently a trivalent, tetravalent orpentavalent cation, 1≦x≦2, 0≦y≦3, 0≦z≦3 and wherein O can be partiallyor completely replaced by divalent and/or trivalent anions such as e.g.N³⁻.